Sexual dimorphism in house sparrow eggs

Transcription

1 Behav Ecol Sociobiol (2000) 48: Springer-Verlag 2000 ORIGINAL ARTICLE Pedro J. Cordero Simon C. Griffith José M. Aparicio David T. Parkin Sexual dimorphism in house sparrow eggs Received: 19 January 2000 / Revised: 29 June 2000 / Accepted 20 July 2000 Abstract Recent evidence has revealed an apparently high degree of control by female birds over the physiological aspects of their reproduction and offspring sex allocation, consistent with adaptive hypotheses of sex allocation and differential investment in their offspring. In the house sparrow, we investigated possible mechanisms that may be used by females to enhance the fitness returns from a reproductive effort. Using molecular techniques, we demonstrate that house sparrow eggs containing male embryos are significantly larger than those containing female embryos. We also found that male embryos were laid randomly with respect to laying order. We speculate that this sexual dimorphism of eggs is adaptive, because male house sparrows show greater variance in condition-dependent reproductive success than females. More important, the result provides further evidence of the ability of females to detect or control ovulation of either male or female ova and to differentially invest in one sex over the other. Key words Egg size Parental investment Passer domesticus Sex ratio Introduction Recent studies have illustrated the ability of female birds to adjust allocation for both the primary and secondary Communicated by M.A. Elgar P.J. Cordero ( ) J.M. Aparicio Departamento de Ecología Evolutiva, Museo Nacional de Ciencias Naturales, C.S.I.C., J. Gutierrez Abascal 2, Madrid, Spain pjcordero@mncn.csic.es Tel.: , Fax: S.C. Griffith Department of Evolutionary Biology, Uppsala University, Norbyvägen 18d, Uppsala, Sweden D.T. Parkin Division of Genetics, Queens Medical Centre, Nottingham, NG7 2UH, UK sex ratio of their clutches, stimulating new empirical research addressing models of adaptive sex allocation in birds (reviewed in Sheldon 1998). Theories of adaptive sex allocation in vertebrates generally rely on differences in the reproductive value of producing sons over daughters (Trivers and Willard 1973; Charnov 1982). If the fitness of the sex with the greatest reproductive variance benefits more from enhanced maternal investment, we would expect to find different patterns of maternal investment in eggs of the two sexes. Differences between eggs bearing sons and daughters will also provide further evidence for the ability of female birds to discriminate internally between male and female ova, and new impetus to the current controversy surrounding maternal effects and differential allocation (Gil et al. 1999; Cunningham and Russell 2000). Recent studies have unambiguously shown biased sex allocation at the primary (ovulation) level in wild bird species (e.g. Komdeur et al. 1997; Sheldon et al. 1999). In addition, there are numerous examples of apparently adaptively skewed secondary sex ratios (some or all of which may be due to primary adjustments). Alternatively, these secondary biased sex ratios may result from differential mortality associated with intra-brood variation in parental investment. For example, several studies have indicated non-random patterns of sex allocation according to the sequence of laying (e.g. Ryder 1983; Weatherhead 1985; Dijkstra et al. 1990; Leroux and Bretagnolle 1996), a phenomenon which may be linked to the observation that first-hatched nestlings may have an advantage over their siblings in terms of the parental care provided (e.g. Bortolotti 1986; Bednarz and Hayden 1991). Females able to provision male and female eggs differentially could make adaptive secondary sex ratio skews (e.g. Mead et al. 1987), assuming that egg provisioning is related to the fitness of hatching offspring (e.g. Nilson and Svensson 1993). Alternatively, egg sexual dimorphism may be an adaptive means by which a female can balance nestling mortality differences between the sexes (through sibling competition) by producing better-provisioned eggs for the disadvantaged sex (Anderson et al. 1997).

2 354 Differential investment in eggs containing either sons or daughters is a phenomenon of great relevance given the current renewed interest in adaptive sex allocation in birds. It will also provide a new perspective on the related fields of sexual selection, sibling competition, and the optimisation of reproductive output, given that there may be differential maternal investment in offspring from ovulation. Here, we investigate patterns of investment in eggs containing embryos of different sex in the house sparrow (Passer domesticus). Primarily, we investigated differences in the size of male- and female-containing eggs. Due to patterns of size variation through the laying sequence of a clutch, we also investigated patterns of egg sex through the laying sequence. In the house sparrow, the male is the sex with higher variance in reproductive success because of their promiscuous behaviour (involving both extra-pair copulation and occasional polygyny; Wetton et al. 1995). The variance in reproductive success is likely to be related to condition either through age dependence (Wetton et al. 1995), or through the condition-dependent sexual ornament (Veiga and Puerta 1996; Griffith et al. 1999a; Griffith 2000). Thus, according to theories of sex allocation and investment (Charnov 1982), we expect male offspring will receive more parental investment (better-provisioned eggs) than their female siblings. We follow the reasonable assumption that there is a positive relationship between egg size and nutrient provisioning (e.g. Parsons 1970; Ricklefs and Marks 1983; Bolton et al. 1992). We also discuss the tactic of modifying egg size according to sex, and its relation to a possible secondary mechanism of sex ratio adjustment. Our data are particularly suitable for this investigation as for each egg we know the exact order in which it was laid and the male who sired it (i.e. within or outside the pair bond; e.g. Cordero et al. 1999a, b). Methods House sparrows were studied between the end of March and early May in 1995 at Cab (Barcelona, Spain) in a nestbox-breeding population in orchards (Salaet and Cordero 1988; Cordero et al. 1999a). Nestboxes were inspected daily and eggs numbered with indelible ink on the day they were laid to determine the precise sequence of laying. The eggs of first breeding attempts were removed from each nest at mid-incubation, measured and frozen. At a later date, the embryos were transferred to absolute ethanol in a proportion of one part of tissue to ten parts ethanol, and stored at 4 C. Further details of field procedures related to assigning social paternity, capturing and sampling adult birds, and measuring body condition and male badge size are given in Cordero et al. (1999a, 1999b). Maximum egg length and breadth from 34 clutches were measured to the nearest 0.05 mm using Vernier calipers. Volume was calculated following the function given by Ojanen et al. (1978). Egg mass was not investigated as it varies during the course of incubation (Ar and Rahn 1980). For comparisons, we considered the relative value between absolute measure of the egg divided by the mean of all eggs in the clutch. The variables obtained in this way are normally distributed and refer to intra-clutch variation. To test for factors accounting for intra-clutch egg size variation, we used MANOVA with the dependent variables relative egg length and breadth, including the independent factors egg order (first, middle-regarded as the mean value for all eggs except the first and the last and last egg) for all available clutches; sex of embryo and female body condition as a covariate. We repeated the same analyses for modal clutch size (5), but in this case the factor egg order was ranked from 1 to 5. As a complementary analysis, we also used the relative volume. A two-factorial ANOVA for relative egg volume was performed for the same two factors as before. We verified that residuals of dependent variables used in ANOVA/MANOVA were normally distributed and the variances were equal between groups. Finally, to further control for the effect of any confounding variables, we performed paired t-tests between average relative size of male and female eggs within clutches. Sex ratio was the proportion of males in a clutch (number of males/total number of eggs). All statistical analyses were conducted using the package SPSS (version ). Sex was determined by PCR amplification of two homologous genes (CHD1-W and CHD1-Z). The CHD1-Z gene occurs in both sexes whereas the CHD1-W gene occurs on the W sex chromosome carried only by females, the heterogametic sex in birds. Following a standard phenol-chloroform extraction of DNA from tissue samples, PCR primers P2 and P8 (Griffiths et al. 1998) were used to amplify from conserved exonic regions across introns of variable length in the two genes (following the PCR protocol of Griffiths et al. 1998). Products were resolved on 6% denaturing polyacrylamide gels. Either one band (CHD1-Z, males) or two distinct bands (CHD1-Z and CHD1-W, females) were seen after silver staining the gels. This technique has been tested using blood from >80 adult house sparrows of known sex in a previous study and all were accurately sexed by this method (S. Griffith, unpublished data). All eggs that were collected from the field were sexed with the exception of 11, in which no embryo had developed and were presumed to be infertile. Molecular determination of sex was carried out by S.C.G. who was blind to the origin of the individual DNA samples. Results The sex ratio in the total sample of eggs was not significantly different from equality (Binomial test, P=1.0). There was no relationship between the sex of the eggs and their rank in the laying sequence, either considering the first and last eggs only (χ 2 =0.22, df=1, P=0.64) or first, middle and last eggs for all clutches (χ 2 =0.76, df=2, P=0.68, n=33). A sex-ratio close to 0.5 was found for all egg orders in the three most common clutch sizes (Fig. 1). A two-factor ANOVA showed that relative egg volume varied according to the sex of the eggs [males Fig. 1 Sex ratio in relation to laying sequence (Egg order) for the most common clutch sizes. Circles clutch size=4 (n=5), triangles clutch size=5 (n=17), open squares clutch size=6 (n=8). Some eggs that were infertile or disappeared (see details in Table 1 in Cordero et al. 1999a) were not analysed and thus are excluded from the graph

3 355 Table 1 MANOVA F-tests for relative egg length and breadth according to egg order, egg sex and their interaction. Female body condition is used as the covariate, all data included. Multivariate F-tests: for interaction between egg order and egg sex, F 4,274 =1.39, P=0.24, effect size=0.02, power test=0.43, for egg order, F 4,274 =5.39, p<0.001, effect size=0.07, power test=0.93; for egg sex, F 2,137 =5.22, P=0.007, effect size=0.07, power test=0.82. Parameter estimates (mean±se) are depicted in Fig. 2 Factor Dependent F df P Eta 2 Power variable test Interaction: Length , order sex Breadth , Egg order Length , Breadth , Egg sex Length , Breadth , Table 2 Intraclutch paired t-test comparisons for average relative sizes of male and female eggs Male Female t 30 P Length 1.004± ± Breadth 1.001± ± Volume 1.007± ± Burley 1988; Petrie and Williams 1993; Cunningham and Russell 2000), we investigated the correlates of egg size and father's badge size, as this sexual trait is an indicator of male attractiveness (e.g. Møller 1990; Griffith et al. 1999b). Father's badge size was not associated with mean relative egg size of sons (r s =0.11 for length, 0.20 for breath and 0.04 for volume; n=22, all P>0.35), of daughters (r s = 0.13 for length, 0.36 for breath and 0.33 for volume; n=21, all P>0.10) or mean egg size of the clutch (r s =0.005 for length, 0.32 for breath and 0.23 for volume; n=23, all P>0.14; Spearman rank correlation tests). Discussion Fig. 2 Egg size (mean±se) in relation to egg order and sex. Full triangles are males, open circles females. All clutches included (n=34) 1.007±0.031, females 0.996±0.036 (mean±sd); F 1,143 =4.07, P=0.045] but neither order of laying (F 2,143 =1.45, P=0.23) nor its interaction with relative egg volume (F 2,143 =1.59, P=0.21) was significant. Controlling for female body condition, a MANOVA showed that the relative length of eggs varied according to both the laying order and sex of the embryo. The interaction between sex and order was almost significant (P=0.07; Table 1). This may be because sex differences in egg size diminished towards the end of the laying sequence. In fact, relative egg length decreased with laying order for males (r s = 0.27, n=73, P=0.019) but not for females (r s = 0.07, n=76, P=0.52). Thus, relative breadth varied only with respect to the laying order, not with sex. Although breadth is not related to egg sex, male eggs were significantly longer than female eggs (Table 1, Fig. 2). This result was maintained when we restricted the analysis to just the modal clutches in the population (five eggs) (P=0.03). To prevent any influence of confounding variables, we performed paired t-tests between average relative length and breadth of male and female eggs within clutches, obtaining again differences in relative length but not in relative breadth between male and female eggs (Table 2). As paternal attractiveness may be important with regard to possible female differential investment (e.g. The differences in egg size (length) obtained indicate that females may allocate resources for egg mass according to the sex of the embryo. Controlling for confounding variables, male embryos develop in relatively larger eggs. The pair-wise analysis of male and female eggs within the same clutch indicated that the sex of the embryo explained 7% of the variance for relative egg length. Laying sequence affects both length and breadth of eggs although relative length is less affected by the order of laying than by the sex of the embryo. In fact, relative egg length decreased with laying order for males, but not for females, which led to no sex-related difference in the last-laid eggs. These results suggest that females were energetically constrained to manage egg provisioning and such limitations were manifested at the end of laying, when a female may have depleted her endogenous reserves and is more constrained for egg laying (e.g. Rydén 1978; Pierotti and Bellrose 1986; Aparicio 1999). The proximate mechanism through which females may adjust the length of the eggs according to the sex of the young is not known, although it must occur after ovulation (sex is determined before ovulation), during the provisioning of the egg. Larger eggs of males than females are also found in another passerine, the whitecrowned sparrow (Zonotrichia leucophrys oriantha) (Mead et al. 1987) and in the American kestrel (Falco sparverius) (Anderson et al. 1997), although the interpretation of the adaptive value of this difference varied. In the white-crowned sparrow, adult males are slightly larger than females and male offspring are also larger and grew faster than their female siblings. Mead et al. (1987) interpreted these results in the light of the Trivers and Willard (1973) hypothesis (but see Teather 1989).

4 356 As male reproductive success exhibits more variance than that of females, female white-crowned sparrows in good condition are expected to facultatively manipulate sex allocation in favour of males. Such an allocation may be facilitated by diverting more resources to male eggs; larger eggs presumably contain more nutrients (e.g. Parsons 1970; Ricklefs and Marks 1983; Bancroft 1984; Bolton et al. 1992; Janiga 1996). As we may expect, there is evidence that larger eggs give rise to heavier nestlings which may in turn lead to increased juvenile survival and recruitment (Nilson and Svensson 1993; Blomqvist et al. 1997; Smith and Bruun 1998; Styrsky et al. 1999). In the American kestrel, in which females are heavier than males, a larger size of male eggs is interpreted as an adaptive compensation in favour of the survival of nestling males in sibling competition, males being the smaller sex (Anderson et al. 1997). The house sparrow is predominantly monogamous but males in good condition may attain a higher reproductive success than females, mostly through polygyny and extra-pair paternity (Wetton et al. 1995; Griffith et al. 1999b). Two studies of the house sparrow have highlighted the importance of good rearing and environmental conditions on the expression of the sexually selected badge of the house sparrow (Veiga and Puerta 1996; Griffith et al. 1999a; Griffith 2000). Extra investment by females in male eggs is therefore likely to be adaptive through the production of sons that are more successful in sexual selection. Previous molecular work by us revealed that all eggs were laid by the socially assigned mothers, and we also identified those eggs sired by extra-pair fathers (Cordero et al. 1999b). Extra-pair paternity did not affect either the sex of the sired offspring (χ 2 =0.10, df=1, P=0.75), or the relative size of the eggs in the clutch (length: F 1,148 =0.27, P=0.60; breadth: F 1,148 =0.32, P=0.57; volume: F 1,148 =0.72, P=0.40). Therefore, the differences between male and female eggs within a clutch are not confounded by parentage and reflect true sexual dimorphism, male eggs being 1.3% larger than female eggs. In house sparrows, as in most other passerines (see Cramp and Perrins 1994), the adult male is slightly larger than the female. Male wing length is approximately 3.0% and the tarsus 3.6% longer in males than in females (data obtained from some breeding pairs of this study: 20 males, 24 females). Some sexual size dimorphism has also been found among siblings, at least in one study in which there was neither differential nestling mortality nor clear evidence for an unbalanced sex ratio at hatching (51% of males) (Schifferli 1980). However, in different wild populations, the operational sex ratio is slightly skewed towards males (53% on average over different populations) (see Summers-Smith 1988). This surplus of males could also be associated with differential post-fledging mortality (Schifferli 1980) because larger fledglings survive better than smaller ones (Ringsby et al. 1998). Small sexual differences in the size of adults, juveniles and eggs, as well as a slightly higher male surplus in the wild suggest that these traits may be associated, but not necessarily causally linked. The sexual differences in egg size found here are probably not enough to provoke substantial differential (by sex) nestling or juvenile mortality, and are therefore unlikely to contribute to secondary biased sex ratios as occurs in some species (e.g. Teather 1989; also Mead et al. 1987). However, these differences in egg size could be associated with final sexual discrepancies in size and survival between male and female house sparrows. If so, sexual egg size dimorphism in the house sparrow could represent real but non-adaptive artefacts of an underlying process not yet identified, as has been previously suggested for other species (Weatherhead 1985; Leblanc 1987). Finally, we found no relationship between female egg provisioning (egg size) and the quality of her male partner (i.e. badge size), suggesting that male attributes may not affect egg size with respect to either male eggs or all eggs in the clutch. While such a relationship may have been predicted by the differential-investment hypothesis (Burley 1988), there are numerous reasons why no such relationship was found. Such differential investment has previously only been found in experimental studies (e.g. Petrie and Williams 1993; Cunningham and Russell 2000) and it may be obscured by the confounding effects of male attractiveness and assortative pairing. We currently do not know the full extent to which females can invest differentiallly in eggs bearing embryos of different sex. In addition to size differences, there are of course other ways in which females can vary their investment in an egg. For example, a recent study of zebra finches apparently revealed differing levels of hormones in eggs fathered by attractive and unattractive mates (Gil et al. 1999). Alongside their study of a captive population using artificial traits of attractiveness, our study underlines the ability of free-living females to make differential investments in eggs. This ability and the maternal effects it generates may have important consequences for many aspects of the evolution of avian life history. Acknowledgements We are grateful to Pere and Josep Cabot for permission to work on their property in Mataró, Barcelona. P.J.C. was attached to CYCIT projects number PB and PB which provided logistic support. The manipulation of the house sparrow population for the whole study, and the removal of first clutches was carried out under license from the Departament d'agricultura, Ramadería i Pesca de la Generalitat de Catalunya, Spain. Thanks to Ben Sheldon for useful discussion and comments on the manuscript. Three anonymous referees and Mark A. Elgar also improved, with their comments and English style, a previous version of the manuscript. References Anderson DJ, Reeve J, Bird DM (1997) Sexually dimorphic eggs, nestling growth and sibling competition in American kestrels Falco sparverius. Funct Ecol 11: Aparicio JM (1999) Intraclutch egg-size variation in the Eurasian kestrel: advantages and disadvantages of hatching from large eggs. 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